In large VCO working range applications (e.g. PLL (Phase Lock Loop)) digital switching of the loading on the VCO ring is used to control the VCO frequency. This switching is used to enable, for example, PLLs to work better. FIG. 3 illustrates such VCO switching loading.
There is a relationship between the frequency and the loading. The relationship of VCO frequency (FVCO) and the loading of VCO ring (CL) will be like this:
  FVCO  ∝            gm              N        ·        CL              .  Where “gm” is equivalence impedance of a ring cell and “N” is the stage number of the VCO ring.
Thus when we turn on/off the switch, the “CL” is changed suddenly, and the “XVCTRL” will not change so fast because of our LOOP CAP, however, the VCO frequency will change suddenly. FIG. 4 shows the VCO frequency change when we turn on the switch (e.g. XS in FIG. 3) at “t0”.
This sudden VCO frequency fluctuation may not be acceptable in real systems because the device following after such a PLL and/or the PLL itself may fail to keep up with the frequency change. This may present a problem.